This invention relates generally to the chemical vapor deposition of coatings onto glass substrates, and more particularly, to an apparatus for the application, by chemical vapor deposition (CVD), of a coating onto glass, especially during its manufacture by the float glass process.
Chemical vapor deposition processes are often used to continuously coat glass substrates while the glass is being manufactured in what is generally known in the art as the float glass process. The float glass process typically involves casting glass onto a molten tin bath which is suitably enclosed, then transferring the glass, after it has sufficiently cooled, to lift-out rolls which are aligned with the bath, and finally cooling the glass as it advances across the rolls through a lehr. The chemical vapor deposition of various coatings may conveniently be performed in the bath, the lehr, or the transition zone therebetween.
Chemical vapor deposition generally involves the formation of a metal, metal compound, or metal oxide coating, or combination thereof, on a surface of a hot glass substrate, by contacting the surface with a gaseous mixture or precursor containing a vaporized substance which undergoes a chemical reaction or decomposition. Such coatings are generally applied to modify the apparent color or solar characteristics of the glass or to impart electrical conductance to the surface of the glass.
The coating or precursor gas is directed onto a surface to be coated by a coater or distributor beam extending across the upper surface of a moving ribbon of glass and transverse to the direction of movement of the ribbon. This device is especially useful for applying a coating from a gas which reacts on contacting the hot glass surface to deposit a coating material on the glass, such as for example a metal vapor. The temperature of the gas supply and the coater is preferably kept sufficiently high to prevent condensation of the coating gas, but sufficiently low to prevent any substantial decomposition or other deleterious reactions of the coating gas before the coating gas reaches the glass surface.
One type of conventional CVD coater may be termed a uniflow coater. In this type of coater, the coating gas is supplied through an elongate passageway from a supply line to an outlet adapted to be positioned adjacent a surface of the glass. The coating gas is directed to the glass surface where it reacts to form the coating. The unreacted portions of the coating gas flow in the direction of glass travel, i.e. downstream, and are drawn away from the glass surface through an exhaust passageway to which is applied a negative pressure.
Generally, the front and back edges of a conventional CVD coater are each superposed over the top surface of the advancing glass ribbon, thereby defining narrow gaps through which gases exterior from the coater may be drawn so as not to contaminate the controlled atmosphere within the float glass facility. However, with a uniflow coater, the upstream edge or toe of the coater must be positioned very near the glass surface to prevent the excessive flow of atmospheric gases into the coating zone, resulting in coating non-uniformity and lower deposition rates.
In response at least in part to these difficulties, dual flow CVD coaters have been developed in which upstream and downstream exhaust passageways are provided on either side of the coating gas supply passageway. Thus, the coating gas is directed through the supply passageway to the glass surface where it reacts to form the coating, and unreacted portions of the coating gas flow both with and against the direction of glass travel, i.e. downstream and upstream, being drawn away from the glass surface through the respective exhaust passageways. Exterior gas which flows under the upstream toe of the coater is drawn up the upstream exhaust passageway along with the upstream flow of unreacted coating gas, so that the upstream toe may be positioned farther from the glass surface.
To enhance the uniformity of the deposited coating, it is generally desired that the flow rate in the upstream reaction zone is balanced with that through the downstream reactions zone. However, the viscous drag caused by the flow of the glass ribbon tends to increase the flow rate in the downstream reaction zone relative to the flow rate in the upstream reaction zone.
One approach utilized in conventional dual flow CVD coaters to compensate for the viscous drag of the glass is to apply a significantly higher negative pressure to the upstream exhaust passageway, thereby increasing the upstream exhaust flow rate. However, depending upon the application and the desired flow rates, the negative pressures required to balance the flow rates in the reaction zones may be difficult to maintain. In addition, such a relatively high flow rate in the upstream exhaust passageway may cause recirculation of the gas, possibly resulting in powder build-up on the coater and haze in the coated glass product.
Another approach which has been utilized in dual flow CVD coaters is to construct the coater asymmetrically, so that there is a significantly greater distance between the supply passageway and the downstream exhaust passageway than between the supply passageway and the upstream exhaust passageway. However, an asymmetrical coater is undesirable from the standpoint of construction flexibility, in that such a coater can be inserted from one side, but not the opposite side, of a float glass manufacturing line due to the supply and control devices which are typically secured to one end of the coater.